Materials that change their electrical resistance in response to a magnetic field may be used in magnetic sensors for a range of applications. For example, such magnetic sensors may be used in a computer to retrieve stored data. In some of those storage devices, data is stored magnetically on a disk. For example, a binary “1” may be stored as a first magnetic state and a binary “0” may be stored as a second magnetic state. When the disk is rotated, a sensor may be brought in close proximity to the rotating disk to determine the magnetic state of the disk at different locations on the disk. The binary ones and zeros sensed by the sensor may be concatenated and provided to a microprocessor as a data stream. The microprocessor may be programmed to interpret the data stream, and use that data to accomplish a task.
There is a desire to have magnetic sensors that are highly sensitive. For example, with respect to data storage devices for computers there is a desire to store more data in the same space. In order to magnetically store more data in a given space, the magnetic zones on the disk may be moved closer together and made smaller. However, if the magnetic zones are moved closer together and smaller, existing sensors may not be able to distinguish one magnetic zone from another, either because the zones are too close or because the magnetic field from a zone is too weak, or both. As a result, there is a desire to have magnetic sensors that are more sensitive.
There are other situations in which it would be desirable to have magnetic sensors that are more sensitive than currently existing devices. For example, it may be desirable to have highly sensitive magnetic sensors employed to detect weak signals from submarines, or to measure variations in the earth's magnetic field.
Around 1988, a so-called ‘giant’ magnetoresistance or GMR effect was discovered. The term “giant” was selected to indicate that a large change in resistance was observed in response to an applied magnetic field. The change in resistance was observed to be from tens to hundreds of percent. The GMR based devices, along with another type of device commonly referred to as Magnetic Tunnel Junctions (“TMJ”), have been applied in the design of read-head sensors for computer data storage devices. For example, the design of Magnetic Random Access Memories (“MRAM”) is influenced by GMR and TMJ. As a result, some MRAMs often include layers of films, each film being a few atoms thick.
In 1999, a large ballistic magnetoresistance (BMR) effect was observed in Ni quantum point conductors. Since then, the effect has been observed in other systems, such as Co and half-metal Fe3O4.